Hysteretic power converter with calibration circuit

ABSTRACT

A power converter includes a first load terminal used to supply a current to a load and a second load terminal used to return a feedback voltage based on the current. A calibration circuit supplies a calibrated voltage processed from the feedback voltage, and a hysteretic comparator controls a current level of the current based on a difference between the feedback voltage and the calibrated voltage.

PRIORITY CLAIM

The present application is a continuation of U.S. application Ser. No. 12/956,630, filed Nov. 30, 2010, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to a power converter, and more particularly to a hysteretic power converter.

BACKGROUND

A power converter, e.g., a direct current (DC) to DC converter, is utilized in many electrical circuit applications, e.g., a Light Emitting Diode (LED) driver circuit. A DC-DC converter includes a hysteretic comparator with a reference voltage as one of its inputs. A controlled voltage of the converter is compared to hysteretic levels, and a high-side switch is turned on if the controlled voltage level is lower than a low threshold point while a low-side switch is turned on if the controlled voltage level is higher than a high threshold point.

However, a conventional DC-DC converter suffers an output offset problem that causes an output current inaccuracy, because of the response time of the hysteretic comparator and/or a loop delay. In an integrated circuit, the current inaccuracy problem becomes worse over process, voltage, and temperature (PVT) variations. For example, the controlled voltage of the converter is supposed to have an average level between two hysteretic threshold points, assuming that the high-side or low-side switch activates immediately when the controlled voltage level crosses a corresponding threshold point.

In a real comparator circuit, there is a response time for the switching action, which causes equivalent threshold levels to deviate. The deviation levels depend on the slew-rate rate of inputs to the comparator as well as PVT. The overall loop delay from output of the comparator corresponding to respective high/low side switching also causes the threshold levels vary with PVT variations. The threshold variation causes inaccuracy of controlled voltage and current.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1A is a schematic diagram showing an exemplary hysteretic power converter (buck converter) with a calibration circuit according to some embodiments;

FIG. 1B is a schematic diagram showing another exemplary hysteretic power converter (buck converter) with a calibration circuit according to some embodiments;

FIG. 2 is a schematic diagram showing yet another exemplary hysteretic power converter (buck converter) with a calibration circuit according to some embodiments;

FIG. 3 is a plot showing waveforms of the exemplary hysteretic power converter (buck converter) in FIG. 2 with a calibration circuit according to some embodiments;

FIG. 4 is a schematic diagram showing yet another exemplary hysteretic power converter (boost converter) with a calibration circuit according to some embodiments; and

FIG. 5 is a flowchart of a method of controlling a power converter, such as one of the exemplary hysteretic power converters in FIG. 1A, FIG. 1B, FIG. 2, and/or FIG. 4 according to some embodiments.

DETAILED DESCRIPTION

The making and using of various embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use, and do not limit the scope of the disclosure.

FIG. 1A is a schematic diagram showing an exemplary hysteretic power converter (buck converter) with a calibration circuit according to some embodiments. The hysteretic power converter 100 includes a hysteretic comparator 102, a calibration circuit 104, a switches S1, a diode 105, an inductor L1, a capacitor C1, a load 106, e.g., light emitting diodes (LEDs), a buffer 107, and a resistor R1. The resistor R1 determines the current level of the load 106. Even though the hysteretic power converter 100 is used as a driver for LEDS in FIG. 1A, different applications for the hysteretic power converter 100 can have different load 106 other than LEDs.

The calibration circuit 104 receives a controlled voltage Vfb (i.e., to control the current of the load 106) as an input and supplies a calibrated voltage Vcomp to the positive terminal of the hysteretic comparator 102 as a variable reference level. The negative terminal of the hysteretic comparator 102 receives Vfb input. The calibration circuit 104 includes a low pass filter (LPF) 108, a comparator 110, a charge pump 116, a capacitor C2, a voltage divider 112, and a voltage clamp 114, and provides the dynamic adjustment of Vcomp. The charge pump 116 includes switches S3 and S4, and current sources I1 and I2. The LPF 108 averages Vfb by filtering high frequency components in the controlled voltage Vfb, which is then compared with Vref1, i.e., the target voltage in the comparator 110. The LPF 108 can be, for example, a resistor-capacitor (RC) filter. The comparator 110 does not have hysteresis, and can be implemented using an op-amp, for example.

As the difference between Vcomp and Vfb, e.g., (Vcomp−Vfb), reaches a high hysteretic threshold level (Vref_high, positive value) or a low hysteretic threshold level (Vref_low, negative value) of the hysteretic comparator 102, i.e. Vfb reaches a low threshold point of (Vcomp−Vref_high), or a high threshold point of (Vcomp−Vref_low) respectively, the switch S1 (and the diode 105) are controlled to keep the Vout and the current through the load 106 at a predetermined level. In one exemplary hysteretic comparator 102, if (Vcomp−Vfb) becomes higher than Vref_high, its output becomes a logical 1. Likewise, if (Vcomp−Vfb) becomes lower than Vref_low, its output becomes a logical 0. The hysteretic comparator 102 output controls the switch S1 (and the diode 105).

For example, if Vcomp minus Vfb is higher than the high hysteretic threshold level (Vref_high), S1 is turned on. When the switch S1 is turned on, a voltage level of Vin is supplied to the inductor L1, the diode 105 is reverse-biased (turned off), and the current through the inductor L1 increases, which also increases Vfb. Thus, if (Vcomp−Vfb) is higher than Vref_high (i.e., Vfb is lower than a low threshold point Vfb_low=Vcomp−Vref_high), Vfb is increased.

If Vcomp minus Vfb is lower than the low hysteretic threshold level (Vref_low), the switch S1 is turned off. When the switch S1 is turned off, a voltage level of Vin is no longer supplied to the inductor L1, the diode 105 is forward-biased (turned on), and the current through the inductor L1 decreases, which also decreases Vfb. The diode 105 functions as a complement switch of the switch S1. Thus, if (Vcomp−Vfb) is lower than Vref_low (i.e., Vfb is higher than a high threshold point Vfb_high=Vcomp−Vref_low), Vfb is decreased.

The comparator 110 of the calibration circuit 104 compares an averaged controlled voltage Vfb (via the low pass filter 108) with a preset target voltage (Vref1) and controls switches S3 and S4. If the averaged controlled voltage Vfb is less than preset voltage the Vref1, the output of the comparator 110 outputs a signal Vcc to the charge pump 116 to close switch S3 (and open switch S4), which increase the voltage Vc of the charge capacitor C2. The Vc passes through a voltage divider 112 and a voltage clamp circuit 114 to supply the divided voltage Vd of Vc as voltage Vcomp to the positive terminal of the hysteretic comparator 102 with a higher level of Vcomp than its previous value. The voltage clamp 114 limits the Vcomp value so that Vcomp does not exceed a predetermined maximum value, e.g., when the hysteretic power converter 100 is at its initial stage starting up.

With the higher Vcomp level, Vfb connected to the negative terminal of the hysteretic comparator 102 swings around a higher reference level of Vcomp in its hysteretic behavior. Thus, the calibration circuit 104 provides a feedback function by adjusting the level of Vcomp based on averaged Vfb compared to Vref1, and compensates a decrease of the averaged controlled voltage Vfb.

Likewise, if the averaged controlled voltage Vfb is greater than the preset voltage Vref1, the output of the comparator 110 outputs a signal Vcc to the charge pump 116 to close switch S4 (and open switch S3), which decreases the voltage Vc of the charge capacitor C2. The Vc passes through a voltage divider 112 and a voltage clamp circuit 114 to supply the divided voltage Vd of Vc as voltage Vcomp to the positive terminal of the hysteretic comparator 102 with a lower level of Vcomp than its previous value. With the lower Vcomp level, Vfb connected to the negative terminal of the hysteretic comparator 102 swings around a lower reference level of Vcomp in its hysteretic behavior. Thus, the calibration circuit 104 provides a feedback function by adjusting the level of Vcomp based on Vfb compared to Vref1, and compensates an increase of the averaged controlled voltage Vfb, by discharging the capacitor voltage Vc and coming to a lower Vcomp at the hysteretic comparator 102.

While Vcomp is calibrated by the calibration circuit 104 as described above, high and low threshold points of the hysteretic comparator 102, i.e., (Vcomp−Vref_low) and (Vcomp−Vref_high), are formed through a hysteretic window around Vcomp from the perspective of Vfb. That means that Vfb swings around both high/low threshold points that are variable and calibrated based on Vcomp, in order to maintain the averaged Vfb equal to the preset value Vref1.

The dynamic and continuous feedback of Vfb to Vcomp by the calibration circuit 104 compensates the offset of threshold points of the hysteretic comparator 102 that may result from variations of response time and loop delay because of the variations of PVT, so that Vfb (that is a portion of the Vout) is controlled to a well-defined/controlled preset value. Thus, the average load current supplied to the load 106 are maintained at a preset value accurately across PVT. For example, without the calibration circuit 104, a lower than expected averaged Vfb and load current (or Vout) value can result from response time and loop delay variations. The controlled current error at the load 106 is well controlled below 1% variation throughout a given range of PVT variations (e.g., various process corners at temperature of −40 to 125° C.), compared to 20% error by another circuit without the calibration circuit 104 under the same operating conditions.

The hysteretic power converter 100 has a fast response to variations of Vfb (or load current) by fast comparison from the comparator 110, as well as current-mode charging/discharging of the charge pump 116. Also, the calibration circuit 104 is compact, thus incurs almost no penalty of chip area. Furthermore, the hysteretic power converter 100 does not have significant adverse effects on power consumption, or efficiency compared to a conventional power converter.

A flexible design can be used in the hysteretic power converter 100, e.g., to control its charging speed, response time (bandwidth), and voltage swing range, etc. For example, if Vref1 is 200 mV, Vcomp may be at some range larger than 200 mV, e.g., 250 mV for a proper operation in some applications with conversion duty cycle larger than 50%. With the voltage divider 112 having a divide value of about ⅕, voltage Vc would be 250 mV×5=1.25 V, so that Vc can be operated in a suitable voltage (e.g., around half of the supply voltage in the charge pump 116) for a convenient design of the current sources I1 and/or I2.

FIG. 1B is a schematic diagram showing another exemplary hysteretic power converter (buck converter) with a calibration circuit according to some embodiments. The hysteretic power converter 101 is implemented using a switch S2 in place of the diode 105 in FIG. 1A. In the hysteretic power converter 101, the hysteretic comparator 102 controls the switches S1 and S2 based on the comparison of the Vcomp and Vfb. As the difference between Vcomp and Vfb, i.e., (Vcomp-Vfb), reaches a high or low hysteretic threshold level of the hysteretic comparator 102 (Vref_high or Vref_low), the switches S1 and S2 are controlled to keep the Vout and the current through the load 106 at a predetermined level.

For example, if Vcomp minus Vfb is higher than the high hysteretic threshold level 1 (Vref_high), S1 is turned on (and S2 is turned off). When S1 is turned on, i.e., closed (and S2 is open), the current through the inductor L1 increases, which also increases Vfb. If Vcomp minus Vfb is lower than the low hysteretic threshold level (Vref_low), S1 is turned off (and S2 is turned on). When S1 is turned off, i.e., open (and S2 is closed), the current through the inductor L1 decreases, which also decreases Vfb. The resistor R1 determines the current level of the load 106. The function of the calibration circuit 104 in the hysteretic power converter 101 is similar to the circuit in FIG. 1A as described above.

FIG. 2 is a schematic diagram showing yet another exemplary hysteretic power converter (buck converter) with a calibration circuit according to some embodiments. In the hysteretic power converters 100 and 101 of FIG. 1A and FIG. 1B, the hysteretic comparator 102 has the high and low hysteretic threshold level (Vref_high and Vref_low) based on the difference between Vcomp and Vfb. Vfb, that are variable and calibrated based on Vcomp, swing around both (Vcomp−Vref_high) and (Vcomp−Vref_low) in order to maintain the averaged Vfb equal to the preset value Vref1.

In comparison, for the hysteretic power converter 200, the two comparators 202 and 204 are non-hysteretic comparators. The calibrated Vcomp supplied to the negative terminal of the comparator 202 functions as a high threshold point in the hysteretic power converter 200. A reference voltage Vref2 is supplied to the positive terminal of the comparator 204 and functions as a low threshold point in the hysteretic power converter 200. The Vref2 is fixed to a preset value, e.g., 150 mV.

The output of the comparator 202 is a signal Sig_stop that enables the switch S2 via a SR-latch 206. The output of the comparator 204 is a signal Sig_start, which enables the switch S1 via a SR-latch 208. For example, if Vfb has a higher level than Vcomp, the Sig_stop signal is a logical 1 (high), while the Sig_start signal is a logical 0 (low). The output Vg2 of the SR-latch 206 would be a logical 1 (high) to turn on the switch S2 and the output Vg1 of the SR-latch 208 would be a logical 0 (low) to turn off the switch S1, respectively. This decreases the level of Vfb. Likewise, if Vfb has a lower level than Vref2, then the opposite of the above happens to increase the level of Vfb. In another embodiment, a similar calibration/dynamic adjustment can be implemented by supplying the calibrated voltage Vcomp to the comparator 204 instead of Vref2, and supplying another reference voltage Vref3 to the comparator 202 instead of Vcomp.

FIG. 3 is a plot showing waveforms of the exemplary hysteretic power converter (buck converter) in FIG. 2 with a calibration circuit according to some embodiments. In this example, Vin=48 V, target Vout=40 V, duty cycle=83% (duty cycle is the time ratio of when the switch S1 is turned on compared to one cycle time), target load (e.g., LED) current=200 mA. Vref1 was set at 200 mV, while Vref2 was set at 150 mV.

Sig_start and Sig_stop waveforms show when the switches S1 and S2 are enabled or disabled. A Vcomp waveform in FIG. 3 shows that Vcomp is continuously calibrated to about 300 mV so that VFB swing by two referenced levels, i.e., 150 mV and 300 mV, to have an averaged 200 mV Vfb despite response time and loop delay variations. The load current waveform shows that the load current is maintained at about 200 mV, while the output voltage (Vout) waveform shows that Vout is maintained at about 40 V.

For a conventional circuit without the calibration circuit 104, the current error was 10%˜20% throughout a given range of PVT variations. For example, in one conventional circuit with the target load current at 200 mV, the load current was 175 mA, or showed 12.5% error. In comparison, one exemplary hysteretic power converter using the calibration circuit 104 showed 1% or less error under the same operating conditions. Also, in one example of the LED driver application, the hysteretic power converter with the calibration circuit 104 performed well when applied to a dimming function (reduced duty cycle). For example, when the duty cycle was 10%, the current accuracy was within 0.25% of the target load current of 20 mA.

FIG. 4 is a schematic diagram showing yet another exemplary hysteretic power converter (boost converter) with a calibration circuit according to some embodiments. The hysteretic power converter 400 is a boost converter, instead of the buck converters in FIG. 1A, FIG. 1B, and/or FIG. 2. The boost converter 400 includes an inductor L2, a switch S5, and a diode 402. When the switch S5 is closed, the current through the inductor L2 increases. When S5 is open, the inductor current flows through the load 106, and Vfb decreases. The calibration circuit 104 functions similarly as described above for the hysteretic power converter 100 (a buck converter) in FIG. 1A.

FIG. 5 is a flowchart of a method of controlling a power converter, such as one of the exemplary hysteretic power converters in FIG. 1A, FIG. 1B, FIG. 2, and/or FIG. 4 according to some embodiments. At step 502, a feedback voltage representing at least a portion of an output voltage, e.g., Vfb, of the power converter, e.g., 100, 101, 200, or 400, is supplied to a calibration circuit, e.g., 104. At step 504, the calibration circuit, e.g., 104, supplies a calibrated voltage, e.g., Vcomp, to a first comparator, e.g., 102 or 202, in the hysteretic power converter, e.g., 100, 101, 200, or 400. At step 506, the first comparator, e.g., 102 or 202, controls the output voltage, e.g., Vout, based on the calibrated voltage, e.g., Vcomp, and the at least a portion of the output voltage, e.g., Vcomp.

In various embodiments, a second comparator, e.g., 110, in the calibration circuit, e.g., 104, receives the feedback voltage, e.g., Vfb. The comparator, e.g., 110, compares the feedback voltage, e.g., Vfb, to a first reference voltage, Vref1. The second comparator, e.g., 110, supplies a calibration comparator output Vcc. The calibration comparator output controls a charge pump, e.g., 116, in the calibration circuit, e.g., 104. The charge pump, e.g., 116, supplies the calibrated voltage, e.g., Vcomp.

In various embodiments, a voltage divider, e.g., 112, in the calibration circuit, e.g., 104, divides an output voltage, e.g., Vc, from the charge pump, e.g., 116. The divided output Vd is supplied as the calibrated voltage, e.g., Vcomp. A voltage clamp, e.g., 114, in the calibration circuit, e.g., 104, limits the calibrated voltage, e.g., Vcomp. A low pass filter, e.g., 108, in the calibration circuit, e.g., 104, averages the feedback voltage, e.g., Vfb.

In various embodiments, a third comparator, e.g., 204, receiving a second reference voltage, e.g., Vref2, and the third comparator, e.g., 204, and the first comparator, e.g., 202, controls the output voltage, e.g., Vout, based on the calibrated voltage, e.g., Vcomp, the feedback voltage, e.g., Vfb, and the second reference voltage, e.g., Vref2.

According to some embodiments, a power converter comprises a first load terminal configured to supply a current to a load and a second load terminal configured to return a feedback voltage based on the current. A calibration circuit is configured to supply a calibrated voltage processed from the feedback voltage, and a hysteretic comparator circuit is configured to control a current level of the current based on a difference between the feedback voltage and the calibrated voltage.

According to some embodiments, a method of controlling a load current of a hysteretic power converter comprises supplying, with a first load terminal, the load current to a load and returning, from a second load terminal, a feedback voltage based on the load current. The feedback voltage is processed with a calibration circuit to generate a calibrated voltage, and a hysteretic comparator circuit controls a current level of the load current based on a difference between the feedback voltage and the calibrated voltage.

According to some embodiments, a power converter circuit comprises a load in series with a resistor and a switch, a capacitor, and an inductor configured to receive an input voltage and supply a current to the load and the resistor, thereby generating a feedback voltage based on the current. A calibration circuit is configured to supply a calibrated voltage based on the feedback voltage, and a hysteretic comparator circuit is configured to control the switch, the capacitor, and the inductor to supply the current at a current level based on a difference between the feedback voltage and the calibrated voltage.

A skilled person in the art will appreciate that there can be many embodiment variations of this disclosure. Although the embodiments and their features have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosed embodiments, processes, machines, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure.

The above method embodiment shows exemplary steps, but they are not necessarily required to be performed in the order shown. Steps may be added, replaced, changed order, and/or eliminated as appropriate, in accordance with the spirit and scope of embodiment of the disclosure. Embodiments that combine different claims and/or different embodiments are within scope of the disclosure and will be apparent to those skilled in the art after reviewing this disclosure. 

What is claimed is:
 1. A power converter, comprising: a first load terminal configured to supply a current to a load; a second load terminal configured to return a feedback voltage based on the current; a calibration circuit configured to supply a calibrated voltage processed from the feedback voltage; and a hysteretic comparator circuit configured to control a current level of the current based on a difference between the feedback voltage and the calibrated voltage.
 2. The power converter of claim 1, wherein the calibration circuit comprises a charge pump configured to supply the calibrated voltage.
 3. The power converter of claim 1, wherein the calibration circuit comprises a comparator configured to control the calibrated voltage based on the feedback voltage and a reference voltage.
 4. The power converter of claim 1, wherein the calibration circuit comprises a low pass filter configured to average the feedback voltage.
 5. The power converter of claim 1, wherein the hysteretic comparator circuit comprises a hysteretic comparator.
 6. The power converter of claim 1, wherein the hysteretic comparator circuit comprises a first non-hysteretic comparator, a second non-hysteretic comparator, a first latch, and a second latch, wherein each of the first non-hysteretic comparator and the second non-hysteretic comparator is configured to receive the feedback voltage, and each of the first latch and the second latch is configured to receive an output of the first non-hysteretic comparator and an output of the second non-hysteretic comparator.
 7. The power converter of claim 1, wherein the load comprises a light emitting diode.
 8. The power converter of claim 1, wherein the feedback voltage is further based on a resistance value of a resistor.
 9. The power converter of claim 1, wherein the power converter further comprises a switch, a capacitor, and an inductor configured to supply the current by increasing an input voltage.
 10. The power converter of claim 1, wherein the power converter further comprises a switch, a capacitor, and an inductor configured to supply the current by decreasing an input voltage.
 11. A method of controlling a load current of a hysteretic power converter, the method comprising: supplying, with a first load terminal, the load current to a load; returning, from a second load terminal, a feedback voltage based on the load current; processing the feedback voltage with a calibration circuit to generate a calibrated voltage; and controlling, with a hysteretic comparator circuit, a current level of the load current based on a difference between the feedback voltage and the calibrated voltage.
 12. The method of claim 11, wherein processing the feedback voltage comprises comparing the feedback voltage to a reference voltage based on a target load current.
 13. The method of claim 11, wherein processing the feedback voltage comprises generating the calibrated voltage using a charge pump.
 14. The method of claim 11, wherein processing the feedback voltage comprises filtering the feedback voltage with a low pass filter.
 15. The method of claim 11, wherein controlling the current level with a hysteretic comparator circuit comprises controlling the current level with a hysteretic comparator.
 16. The method of claim 11, wherein controlling the current level with a hysteretic comparator circuit comprises controlling the current level with a first non-hysteretic comparator, a second non-hysteretic comparator, a first latch, and a second latch, wherein each of the first non-hysteretic comparator and the second non-hysteretic comparator receives the feedback voltage, and each of the first latch and the second latch receives an output of the first non-hysteretic comparator and an output of the second non-hysteretic comparator.
 17. The method of claim 11, wherein supplying the load current to the load comprises supplying the load current to a light emitting diode.
 18. A power converter circuit, comprising: a load in series with a resistor; a switch, a capacitor, and an inductor configured to receive an input voltage and supply a current to the load and the resistor, thereby generating a feedback voltage based on the current; a calibration circuit configured to supply a calibrated voltage based on the feedback voltage; and a hysteretic comparator circuit configured to control the switch, the capacitor, and the inductor to supply the current at a current level based on a difference between the feedback voltage and the calibrated voltage.
 19. The power converter circuit of claim 18, wherein the calibration circuit is further configured to supply the calibrated voltage based on a reference voltage corresponding to a target current and a resistance value of the resistor.
 20. The power converter circuit of claim 18, wherein the load comprises a light emitting diode. 